An early sign of magnetron failure is its beginning to mode, that is, to abruptly cease to conduct anode current. Since the majority of magnetron power supplies are of the current limiting type each interruption of anode current results in a large voltage transient. Herefore, direct observation of this circuit phenomenon required the use of expensive and sophisticated laboratory equipment such as high frequency oscilloscopes. Because service personnel diagnosing and repairing problems in microwave ovens are customarily not trained in the use of and do not ordinarily have access to laboratory oscilloscopes while performing service on microwave ovens, direct observation of magnetron moding has been previously unavailable as a procedure in the servicing of microwave ovens.
Heretofore, the most common method of diagnosing magnetron moding in the servicing of microwave ovens has been by observing a reduction in the apparent power output of the microwave oven. However, diagnosing a "defective" magnetron by this method has proven unreliable and expensive since the symptom may be indicative of other problems than magnetron moding (as for example, low line voltage) and also has led to the premature replacement of magnetrons which have a significant remaining useful life, even though they may exhibit infrequent moding.